As you can see from other Linux OS, gtk3 uses file names different from gtk2.
I.e. both versions can easily be installed at the same time.
Example, gtk-3.2.3 :
/usr/lib/libgdk-3.so.0
/usr/lib/libgdk-3.so.0.200.3
/usr/lib/libgtk-3.so.0
/usr/lib/libgtk-3.so.0.200.3 , etc. etc.
http://rpm.pbone.net/ >
> Advanced Search : Fedora 16 , keyword = gtk3
.